Preparation of the Isomeric Thienothiophenes and Related Systems

According to the available literature, two main routes to the synthesis of isomeric thienothiophenes have been studied cyclization of various aliphatic substances, and cyclization of derivatives of the thiophene series. 

Soon after thiophene and its derivatives had been prepared by treating 1,4-dicarbonyl compounds with P2S3, Biedermann and Jacobson extended this procedure to citric and tricarballylic acids. By heating a mixture of citric acid and PjSj, they obtained a compound CJH4S2, b.p. 224°-226°, in about 1% yield. The structure 1 was ascribed to this compound, which was called thiophthene [Eq. (1)]. 

Similar results were obtained from the sodium salt of tricarballylic acid with PjSj. 

In 1904 Oster obtained 1 in slightly higher yield by adding 10% of dry quartz sand to the mixture of citric acid and PjSj. Hanna and Smith also prepared 1 by allowing PjS, to react with sodium aconitate, but the yield remained very low. 

A synthetic route developed by Capelle and deConinck in 1908 produced thienothiophene 1 in somewhat better yields. By passing acetylene through the vapors of boiling sulfur and then trapi g the reaction products by carbon disulfide, they isolated, together with benzo[bl-thiophene, a compound, b.p. 225°, which formed a picrate similar to that obtained by Biedermann and Jacobson.  

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